How to iterate a map in reverse order – C++

In this article we will discuss how to iterate a map in reverse order.

Map store the elements in sorted order of keys. For example, if you store following elements in map i.e.

{"aaa", 10},
{"ddd", 11},
{"bbb", 12},
{"ccc", 13}

It will internally store the elements in sorted order of keys i.e.

{"aaa", 10},
{"bbb", 12},
{"ccc", 13},
{"ddd", 11},

Iterating over a map will result in above order i.e. sorted order of keys. Now what if we want to iterate a map in reverse direction ?

Iterating a map in reverse order

To Iterate a map in reverse order we will use reverse_iterator of map i.e.

Reverse Iterator

std::map<std::string, int>::reverse_iterator

Reverse Iterator of map moves in backward direction on increment. So, we will point the reverse_iterator to the last element of map and then keep on incrementing it until it reaches the first element. To do this we will use 2 member functions of std::map i.e.

std::map::rbegin()

reverse_iterator rbegin();

It returns the reverse_iterator pointing to last element of map.

std::map::rend()

reverse_iterator rend();

It returns the reverse_iterator pointing to first element of map.

Now, to loop back in reverse direction on a map. Iterate over the range b/w rbegin() & rend() using reverse_iterator.

Checkout complete example as follows,

#include <iostream>
#include <map>
#include <string>
#include <iterator>

int main() {

	// Creating & Initializing a map of String & Ints
	std::map<std::string, int> mapOfWordCount = {
			{ "aaa", 10 },
			{ "ddd", 11 },
			{ "bbb", 12 },
			{ "ccc", 13 }
	};

	// Create a map iterator and point to the end of map
	std::map<std::string, int>::reverse_iterator it = mapOfWordCount.rbegin();

	// Iterate over the map using Iterator till beginning.
	while (it != mapOfWordCount.rend()) {
		// Accessing KEY from element pointed by it.
		std::string word = it->first;

		// Accessing VALUE from element pointed by it.
		int count = it->second;

		std::cout << word << " :: " << count << std::endl;

		// Increment the Iterator to point to next entry
		it++;
	}

	return 0;
}

Output:

ddd :: 11
ccc :: 13
bbb :: 12
aaa :: 10

To compile the above code on linux. Use following command,

g++ -std=c++11 example.cpp